Rate constants for ligands interacting with cell bound or dispersed receptors have a diffusive part and an intrinsic part: the former depending on geometry, receptor distributions, and diffusion coefficients; the latter on electronic redistributions. We have been focusing on the former and have obtained expressions for diffusion limited association and dissociation rate constants when (1) ligands bind directly and specifically to receptors that are distributed over a spherical surface; (2) ligands bind indirectly by a path that includes non specific association with the cell and diffusion in the surface, toward or away from a specific receptor. We have also developed a formalism that permits calculation of the complete equilibrium and rate constants for cell bound receptors, given the equilibrium or rate constants for dispersed receptors. Mathematical methods are also being developed to describe aggregation on a two dimensional fluid surface.